Various electromagnetic (EM) survey techniques exist to perform surveys of a subterranean structure for identifying subterranean elements of interest. Examples of subterranean elements of interest include hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One type of EM survey technique is the magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another type of EM survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an EM transmitter, called a “source,” is used to generate EM signals. With either EM survey technique, surveying units, called “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structure can be derived. The EM receivers may include a number of sensors for detecting any combination of electric fields, electric currents, and magnetic fields.
In a marine environment, EM sources are typically towed by a marine vessel through a body of water. Conventionally, EM receivers are deployed on the sea floor. However, it can be labor and time intensive to deploy and recover EM receivers deployed on a sea floor.
Moreover, a further issue associated with certain EM sources is that they tend to excite two modes of EM fields: a transverse magnetic (TM) mode and a transverse electric (TE) mode. Typically, EM fields according to the TM mode are sensitive to thin resistors in the subterranean structure, while EM fields according to the TE mode are insensitive to the thin resistors in the subterranean structure. Unfortunately, the TE mode that is insensitive to thin resistors often dominates the measured response. In addition, the resulting response based on the processing of the measured EM data contains not only the scattered field that contains information regarding the subterranean structure, but also a “primary” field that contains no information about the subterranean structure, which can lead to inaccurate results.